Post-treatment strategies for pyrophoric KOH-activated carbon nanofibres

The effect of two atmospheric post-treatment conditions directly after the KOH activation of polyacrylonitrile-based nanofibres is studied in this work. As post-treatment different N2 : O2 flow conditions, namely high O2-flow and low O2-flow, are applied and their impact on occurring reactions and carbon nanofibres' properties is studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), Raman spectroscopy, elemental analysis and CO2 and Ar gas adsorption. At high O2-flow conditions a pyrophoric effect was observed on the KOH-activated carbon nanofibers. Based on the obtained results from the TGA and DSC the pyrophoric effect is attributed to the oxidation reactions of metallic potassium formed during the KOH activation process and a consequent carbon combustion reaction. Suppression of this pyrophoric effect is achieved using the low O2-flow conditions due to a lower heat formation of the potassium oxidation and the absence of carbon combustion. Compared to the high O2-flow samples no partial destruction of the carbon nanofibers is observed in the SEM images. The determination of the adsorption isotherms, the surface area, the pore size distribution and the isosteric enthalpies of adsorption show the superior properties under low O2-flow conditions. The present micropore volume is increased from 0.424 cm3 g−1 at high O2-flow to 0.806 cm3 g−1 for low O2-flow samples, resulting in an increase of CO2 adsorption capacity of 38% up to 6.6 mmol g−1 at 1 bar. This significant improvement clearly points out the importance of considering highly exothermic potassium oxidation reactions and possible post-treatment strategies when applying KOH activation to electrospun carbon nanofiber materials.


Introduction
2][3][4][5][6] Amongst others, activated carbons are a frequently used material class due to their high micropore volume and high specic surface area.They are synthesized via a two-step process, comprised of carbonisation and activation.8][9] The applied process and activation method depends on the desired surface morphology and porosity of the carbon for the individual application.Therefore, a variety of activation processes have been developed to optimise the pore structure of carbons, which can be divided into physical activation and chemical activation methods.Physical activation is done using oxidising gases, e.g.CO 2 or H 2 O, whereas chemical activation uses solid activating agents, e.g.1][12] As a chemical activating agent for carbonbased materials, KOH is well-established and was patented by Wennerberg et al. in 1978. 13Since then, KOH activation has been widely used to activate many different precursor materials, e.g.coals, biomass and carbon bres. 1,12,14KOH is mainly applied to obtain a high micropore volume and a high specic surface area of the activated materials.Frequently reported BET areas for KOH activated carbon bres range from 1000 up to 3000 m 2 g −1 and the total pore volume easily reaches up to 2 cm 3 g −1 . 9,15Despite the widespread application for decades, the exact mechanism of the KOH activation is still discussed.[17] 6KOH + 2C 4 2K + 3H 2 + 2K 2 CO 3 , DH R = +428 kJ mol −1 (1) 9][20] The dehydration of KOH resulting in K 2 O and H 2 O (eqn (2)) is proposed as starting reaction for the KOH activation of petroleum coke. 15,16On elevating the temperature, the carbon and the H 2 O are transformed into H 2 and CO via a coal gasication reaction (eqn (3)).Additional H 2 O reacts via the water gas shi reaction with the formed CO resulting in H 2 and CO 2 (eqn (4)).The obtained K 2 O (eqn (2)) and CO 2 (eqn (4)) are transformed to K 2 CO 3 at temperatures between 400 and 800 °C (eqn (5)).Furthermore, at reaction temperatures >700 °C K 2 O is reduced by H 2 and carbon to metallic potassium (eqn ( 6) and ( 7)), which can intercalate into the carbon lattice. 16OH 4 K 2 O + H 2 O, DH R = +245 kJ mol −1 (2) CO + H 2 O 4 H 2 + CO 2 , DH R = −41 kJ mol −1 (4) Similar reactions were proposed for the KOH activation of multi-walled carbon nanotubes with the formation of K 2 CO 3 via redox reactions starting at 400 °C. 17In subsequent reactions the K 2 CO 3 etches the carbon framework and K 2 O and CO are formed (eqn (8)).
Nevertheless, the actual reaction pathway highly depends on the applied activation parameters and the used precursor.Moreover, the structure of the precursor also affects the possibility of potassium intercalation.Overall, these impact factors render the assessment of the exact occurring reaction mechanisms a difficult task. 1,20ince the rst patents, countless studies have been published on the impact of the different activation parameters, e.g.activation temperature, KOH : precursor ratio and activation duration. 20,21In contrast, the impact of the atmospheric posttreatment conditions directly subsequent to KOH activation has not been investigated so far.However, this factor may have a serious impact on the obtained activated carbon due to the high reactivity of elemental potassium formed during KOH activation.Elemental potassium is known to react vigorously in the presence of oxygen and water due to the formation of potassium oxides according to eqn ( 9)- (11). 22,23 þ The occurrence of such reactions in potassium-treated carbon materials has been frequently reported and was rst observed by Fredenhagen and Cadenbach, who obtained pyrophoric potassium-graphite intercalation compounds in 1926. 24,25Additionally, a similar pyrophoric effect is described for the intercalation products of coals and amorphous carbons with K 2 CO 3 during K 2 CO 3 -catalyzed gasication. 26Such an intercalation of potassium from melts into graphite and carbons was detailed studied in several publications. 25,27,28he present work investigates, the occurrence of such a pyrophoric effect aer KOH activation by application of two different atmospheric post-treatments.0][31][32][33][34] TGA-MS, DSC, SEM, elemental analysis, Raman spectroscopy and gas adsorption techniques are used to investigated occurring reactions and changes to the bre morphology and porosity.

Carbon nanobres synthesis
For the synthesis of the carbon nanobres, solutions of 10 wt.% PAN (150 000 g mol −1 , BOC Science, USA) in N,N-dimethyl formamide (VWR Chemicals, Germany) were prepared.All chemicals were used as received without further purication.To obtain a complete dissolution the mixtures were stirred for 48 h at room temperature.Subsequently, the solution was electrospun using an electrospinning device (IME Technologies, Netherlands).The polymer solution was supplied at a ow rate of 120 mL min −1 through a 4-tip spinning needle.The spinneret was moved laterally to the collector drum with a speed of 20 mm s −1 and a turn delay of 500 ms within a range of 120 mm.The spinning process was conducted at constant climate conditions of 25 °C and 30% relative humidity and the applied voltage was set to 25 kV.The tip-collector distance was 120 mm.The bres were collected on a rotating drum with a diameter of 90 mm and a rotational speed of 1000 rpm.In total, the spinning was conducted for 3 h, which corresponds to 21.6 mL used spinning solution.Subsequently, oxidative stabilization at 250 °C in air for 15 h was performed at a heating rate of 5 K min −1 using a drying cabinet (Binder GmbH, Germany).In the following step, 200 mg of the stabilized nanobres were impregnated with 10 mL aqueous KOH (c = 0.26 mol L −1 , KOH : PAN weight ratio 3 : 4) for 2 h.Aerwards, the sample was dried at 85 °C for 3 h to remove H 2 O.
The entire carbonisation & activation process was conducted inside a thermogravimetric analyser (STA 449 F1 Jupiter, Netzsch GmbH, Germany) coupled to a mass spectrometer (QMS 403 D Aëlos, Netzsch GmbH, Germany) (TGA-MS).200 mg of the obtained KOH impregnated nanobres were transferred into the 5 mL TGA beaker to conduct the simultaneous carbonisation and activation.The TGA furnace was purged trifold with Ar to ensure an inert carbonisation atmosphere.Aerwards, the samples were heated at a rate of 300 K h −1 up to 800 °C and held for 3 h at this temperature in inert atmosphere at a ow rate of 40 mL min −1 Ar 5.2 (Air Liquide, France).Aerwards, the samples were cooled down to 40 °C at a rate of 200 K h −1 .Once a temperature of 40 °C was reached, the atmosphere was changed to an O 2 : N 2 atmosphere.The used O 2 -ow rates were 4 mL min −1 for low O 2 -ow conditions and 175 mL min −1 for high O 2 -ow conditions.The respective N 2 ow rates were 16 mL min −1 for low O 2 -ow conditions and 75 mL min −1 for high O 2 -ow conditions.Aer the switch to O 2 : N 2 atmosphere, the samples were kept for 45 minutes under the applied atmospheric conditions.Aer this process steps, the carbon nano-bres were obtained and neutralized using distilled water in several washing steps until a neutral pH value was achieved.Finally, the samples were dried at 100 °C.For each posttreatment, a vefold determination was carried out.
Additionally, similar experiments were conducted in a horizontal tube furnace (REST-E 400/6, Carbolite Gero GmbH & Co. KG, Germany).200 mg of the KOH impregnated sample were transferred into an alumina boat and the furnace was trifold purged with Ar.Subsequently, the sample was heated up to 800 °C at a heating rate of 300 K h −1 in inert atmosphere at a ow rate of 105 L h −1 Ar 5.2 (Air Liquide, France).The temperature was maintained for 3 h and aerwards cooled down at a rate of 200 K h −1 .At a temperature of 40 °C the furnace was opened and the samples were exposed directly to ambient air, which equals high O 2 -ow conditions.

Material characterization
Mass and temperature changes, as well as the gaseous reaction products were detected by the TGA-MS system.The MS measurements were conducted in multiple ion detection (MID) mode.Relevant gases were identied with a scan measurement for m/z 0-100 in an additional run prior to the actual measurements.The obtained MID runs were normalized to a reference run, to correct contamination of the device caused by the formation of soot particles during the reaction.
For data evaluation, the peak area for the detected components was determined by integration.The obtained weight normalized peak area is converted into the molar amount of CO 2 based on a calibration using CaC 2 O 4 × H 2 O.For the calibration masses of 50, 75, 100, 125 and 150 mg of CaC 2 O 4 × H 2 O were transferred into the TGA crucible and aerwards heated to 1000 °C.Based on the obtained results, a relation of peak area CO 2 to molar amount could be drawn (S1).† 35 Additionally, differential scanning calorimetry (DSC) measurements were performed on a STA 449 F1 Jupiter (Netzsch GmbH, Germany) to measure the released reaction heats.Prior to the measurements, a calibration using a sapphire disc was performed.The DSC measurements were conducted with an equal heat treatment as for the carbonisation and activation process in the TGA beaker.The sample mass of KOH impregnated nanobres for these measurements was 20-30 mg.For each post-treatment condition, a triple determination was conducted.
Elemental analysis was conducted using a varioELcube elemental analyser (Elementar, Germany).A triple determination with 2 mg for each sample was carried out to determine C, H and N content.The O content was determined as the difference between the CHN content to the total composition.
Inductively coupled plasma optical emission spectroscopy (ICP-OES) was performed on iCAP 7600 analyser (ThermoFisher Scientic, USA).Two parallel digestions of 100 mg sample each were prepared in a furnace with 250 mg lithium borate.The samples were heated to 1050 °C during 3 hours and maintained for 30 minutes at this temperature.The obtained melt was diluted in 50 mL 5% HNO 3 and lled up to 100 mL.
A Quanta FEG 650 microscope (FEI, USA) was used to conduct the scanning electron microscopy (SEM) investigations.For image recording an acceleration voltage of 20 kV and an Everhart-Thornley detector was used.Small parts of the sample were xed on the sample holders using copper strips.
Gas adsorption measurements were performed on a 3P micro 300 (3P Instruments, Germany).Argon (5.2, Air Liquide, France) adsorption measurements were conducted at 87 K and CO 2 (4.5, Air Liquide, France) adsorption measurements at 273 K. Prior to the measurements the samples were outgassed at 150 °C for 12 h.
The obtained data was evaluated using Asiqwin 5.0 (Quantachrome Instruments, USA).The adsorption isotherms were tted as a Tóth isotherm 36 to calculate a mean isotherm for each parameter set.As error assessment the standard deviation was determined.A quenched solid state DFT (QSDFT) equilibrium model was applied to calculate the pore size distribution of the Argon adsorption data using a slit pore model on carbon.The pore size distribution of the CO 2 adsorption isotherms was obtained by a Monte-Carlo model on carbon for slit pores.
Similar to the isotherm ts a mean value was calculated and the error indicated by the standard deviation.Additionally, the determination of the BET area, taking into account its limitations for microporous materials, was performed. 37,38etermination of the adsorption kinetics was performed using an Autosorb iQ 2 (Quantachrome, USA) by measurement of a single point isotherm at 50 mbar at 298 K using the VectorDose™ mode.
For calculation of the isosteric enthalpy of adsorption, CO 2 adsorption isotherms were measured at 273 K, 283 K and 293 K on a QuadraSorb EVO (Quantachrome, USA) and tted using a Tóth Fit.The calculation of the isosteric enthalpies of adsorption was conducted via the Clausius-Clapeyron approach. 39

Results and discussion
In preliminary experiments KOH-activated carbon nanobers were post-treated at ambient air as it is the usual procedure. 1,11,12During the exposure to ambient air, a strong red glowing of the carbon nanobres was observed, which indicates vigorous potassium oxidation reactions (Fig. 1 and Video in the ESI †).
Detailed investigations on this pyrophoric effect using a TGA-MS following the heat treatment in Fig. 2 were conducted to get a deeper insight into the occurring reactions.Therefore, two different post-treatment conditions were applied subsequent to the simultaneous carbonisation and activation: one, labelled as 'low O 2 -ow' at an O 2 -ow rate of 4 mL min −1 (N 2 : O 2 80 : 20) and a second one at 175 mL min −1 (N 2 : O 2 30 : 70), denoted as 'high O 2 -ow'.Additionally, SEM, DSC, elemental analysis, Raman spectroscopy and gas adsorption were used to obtain detailed information on the observed glowing, the reaction process and its impact on the bre morphology.

High O 2 -ow
In Fig. 3a the mass and temperature changes of the freshly activated samples on switching the atmosphere from Ar to a mixture of N 2 : O 2 (30 : 70) are shown for the high O 2 -ow.The vertical dashed line indicates the switch from Argon to N 2 : O 2 (30 : 70) atmosphere at 175 mL min −1 O 2 -ow.Almost instantly aer the change in atmosphere, a mass gain of 2.8 ± 0.7 wt.% is observed.Aerwards, a mass loss of 3.1 ± 1.1% occurs before the mass slightly increases again.Simultaneously, the temperature rises to 130 °C and cools down again to the temperature set point of 40 °C aerwards.However, it must be emphasized, that this is only an apparent temperature, as the reaction heat is partially consumed by the heat capacity of comparatively large TGA crucible.The actual temperature of the sample during the reaction is signicantly higher.
The mass gain and the temperature rise can be assigned to the oxidation reactions of elemental potassium, which was formed during the KOH activation reactions (eqn ( 6) and ( 7)). 22,23,40The formation of K 2 O (eqn (9)), K 2 O 2 (eqn (10)) and KO 2 (eqn ( 11)) releases high reaction enthalpies.The oxidation reactions most probably result in the formation of KO 2 for conditions with high availability of oxygen, due to the higher lattice stability compared to K 2 O and K 2 O 2 . 22,23he mass gain of 2.8 ± 0.7 wt.% corresponds to the O 2 uptake of 0.87 mmol g −1 , which would require a potassium amount of 0.87 mmol g −1 according to eqn (11).This corresponds to 14% of the total potassium, present at the carbon nanobre samples, which was determined via ICP-OES (S2).† The calculated reaction heat of the formation of 0.87 mmol g −1 KO 2 is −248 J g −1 according to eqn (11), which is sufficient to cause a signicant temperature rise.
However, these reactions cannot explain the subsequent mass loss of 3.1 ± 1.1 wt.%.An analysis of the gaseous products determined CO 2 as main emission (S3), † suggesting a carbon partial combustion reaction caused by the reaction heat of the potassium oxidation.A total carbon loss of 2.62 mmol g −1 is calculated via eqn (12) accompanied by −1020 J g −1 reaction heat.In total potassium oxidation and carbon combustion reactions result in an emitted reaction heat of −1268 J g −1 .
Comparing the calculated reaction heats of KO 2 and CO 2 formation, the emitted reaction heat is caused to almost 80% by the carbon combustion to CO 2 .According to the basic heat equation of thermodynamics, the emitted heat could cause a temperature increase to a temperature higher than 700 °C, which would explain the observed glowing of the carbon nanobres (Fig. 1).
Based on the calculated carbon loss the theoretical released amount of CO 2 would be 2.58 mmol g −1 , which equals a CO 2  release of 114 mg g −1 .The amount of CO 2 determined via quantication of the gaseous products is lower, giving a value of 40.5 mg g −1 .The signicant difference could be explained by the emission of solid particles due to the vigorous reaction, which are not detected by the MS.Additionally, side reactions due to formed KO 2 are possible during the reaction as listed in eqn ( 13) and ( 14).The possible side reactions mainly result in the formation of potassium carbonate and bicarbonate. 41,42O 2 + CO 2 4 K 2 CO 3 + 1.5O 2 , DH R = −186 kJ mol −1 (13) Furthermore, combined TG-DSC measurements were conducted to obtain experimental data on the emitted reaction heats for high O 2 -ow (Fig. 3b).The simultaneous TG measurement was done to assess the comparability of both measurements since different sized crucibles had to be used for both measurements.An emitted reaction heat of −2717 J g −1 was determined for the post-treatment step at high O 2 -ow (Fig. 3b).This value is about twice as high as the calculated reaction heat of −1268 J g −1 from previous TG measurements (Fig. 3a).
The deviation of these two methods is explicable, when considering the shape of the TG signal of the DSC measurement.Similar to the rst discussed TG signal, a mass gain is observed directly aer the switch to the high O 2 -ow.Subsequently, a signicant mass loss due to carbon combustion occurs.However, the mass loss during the DSC measurement is 8.2 wt.%, which is more than double the value of 3.1 wt.% during the TG measurement.Due to the increased carbon combustion during the DSC measurement the obtained experimental value and the calculated reaction enthalpy from the TG measurement may not be directly comparable.The increased carbon combustion is probably caused by denser packed carbon nanobres in the smaller DSC crucible, resulting in a stronger heat accumulation.This is an important hint, as the extent of potassium oxidation and carbon combustion reactions depend on the packing density and, therefore, heat and oxygen transport properties of the material.
Interestingly, similar observations are not described for KOH activation of commonly used carbon precursors, e.g.PAN powder, 43,44 carbon bres, 45-47 biomass 48-50 and coals. 18,51,52lthough, the occurrence of these potassium oxidation reactions could be expected based on the proposed mechanisms for KOH activation mentioned in eqn ( 6) and (7).Therefore, PAN powder was used as reference material and activated using the identical conditions as for activation of the nanobre material.During high O 2 -ow post-treatment no pyrophoric effect was observed (S4).† Hence, the described pyrophoric effect is probably linked to the material structure induced by the electrospinning process and, possibly, the activation conditions.
Overall, the post-treatment at high O 2 -ow conditions results in the uncontrolled oxidation of metallic potassium to KO 2 causing a carbon combustion under severe formation of heat and a strong glowing of the carbon nanobres.

Low O 2 -ow
To develop a treatment strategy to suppress the sample glowing, similar experiments were conducted using an O 2 -ow of 4 mL min −1 , 'low O 2 -ow'.Fig. 4a shows the mass and temperature changes aer the switch of atmosphere from Ar to a mixture of N 2 : O 2 (80 : 20) with an O 2 -ow rate of 4 mL min −1 , indicated by the dashed line.
A mass gain of 1.5 ± 0.1 wt.% accompanied by a minor temperature rise to 45 °C aer 8 minutes is observed aer the atmosphere switch.Contrary to the high O 2 -ow, no weight loss is observed.The mass gain can be explained by the uptake of O 2 due to the oxidation of metallic potassium according to eqn (9)- (11).As the temperature increase is insufficient to cause a carbon combustion, no mass loss is observed.The obtained mass gain equals an O 2 uptake of 0.44 mmol g −1 , based on eqn (12) this results in the emission of −128 J g −1 reaction heat.This calculated reaction heat is comparable to the experimental value of −94 J g −1 obtained from additional DSC measurements (Fig. 4b) and causes a slight temperature rise on the carbon nanobres.The slight deviation of the obtained reaction heats can be explained by the simple nature of the calculations used, which only considered one potassium formation reaction neglecting possible side reactions to the formation of other potassium oxide species. 22,23,41ummarising, the low O 2 -ow post-treatment results in the controlled oxidation reactions during exposure to oxygen.The emitted reaction heat of −94 J g −1 is signicantly reduced compared to −2717 J g −1 for high O 2 -ow.Therefore, no indication for a carbon combustion or glowing was found.

Comparison of high O 2 -ow and low O 2 -ow
Besides the previously discussed mass and temperature changes during the sample treatment with high O 2 -ow and low O 2 -ow, this section addresses differences in the emitted CO 2 amount, elemental composition and morphological changes observed in SEM.
3.3.1 Mass balances.Fig. 5 shows the emitted amount of CO 2 in mmol g −1 for high O 2 -ow and low O 2 -ow as measured by the MS.The detected CO 2 amount for high O 2 -ow is at 1000 mmol g −1 , whereas the CO 2 emission for low O 2 -ow was at 0.6 mmol g −1 .This means a reduction of the emitted CO 2 amount of more than three orders of magnitude for low O 2 -ow conditions.These results match with the absence of a mass loss for low O 2 -ow discussed in the previous section.
Additionally, CHNO elemental analysis was conducted to detect changes in the elemental composition of the obtained carbon nanobres for high and low O 2 -ow (Table 1).The C content for low O 2 -ow is 69.6 wt.%, whereas it is at 59.8 wt.% for high O 2 -ow.This 10 wt.% decreased C content can be related to the carbon combustion and possibly also to oxidation of the carbon surface.(Fig. 6).For reference purposes the pristine bres 30 are shown in Fig. 6a as well.
The pristine carbon nanobres are randomly aligned and large void volumes are visible between the bres.For the KOH activated carbon nanobres smaller void volumes and fragmented bres are observed.The low O 2 -ow samples exhibit a similar morphology as the pristine material (Fig. 6b, S6 † vs. 6a).In contrast, high O 2 -ow samples (Fig. 6d) exhibit areas with signicant destruction of surface morphology compared to the pristine material (Fig. 6a).The formation of voids in the size of up to 3 microns is clearly visible in the SEM images (Fig. 6d).These macropores are not homogeneously distributed on the surface of the high O 2 -ow samples as also areas with less severe destruction are visible (Fig. 6c).This observation could be explained by an inhomogeneous distribution of metallic potassium on the samples and, therefore, inhomogeneous severe heat formation due to the potassium oxidation reactions.
Summarising, the obtained SEM results prove that the control of post-treatment conditions signicantly affects the morphology of the samples.The applied O 2 -ow affects the potassium oxidation reactions and a severe destruction of the carbon nanobres can be avoided by application of a low O 2 -ow as post-treatment.

Gas adsorption properties.
To obtain detailed information on the impact of the different post-treatment conditions on the pore structure and the adsorption properties, gas adsorption measurements were conducted for high O 2 -ow and low O 2 -ow samples.
Argon adsorption measurements were performed at 87 K to assess the microporosity.The obtained adsorption isotherms exhibit a type I shape, which is typical for highly microporous adsorbents (Fig. 7a).In the low relative pressure range a steep increase is observed, which attens for higher relative pressures and approaches a limiting value.The obtained isotherms exhibit similar shapes independent of post-treatment, although the obtained adsorption capacities are signicantly higher on the low O 2 -ow samples.At 1 bar 17 mmol g −1 CO 2 are adsorbed on the low O 2 -ow samples, whereas 31 mmol g −1 CO 2 are obtained for high O 2 -ow.This equals an increase in adsorption capacity at 1 bar by 82%.
The pore size distribution was derived from the Ar adsorption isotherms using a density functional theory (DFT) kernel.The cumulative pore size distributions for low O 2 -ow and high O 2 -ow are shown in Fig. 7b.The comparison of the total pore volume V DFT(tot) and the pore volume of pores below 2 nm (V DFT<2nm ) reveals that both samples have almost exclusively micropores (Table 2, V DFT<2nm ).The obtained micropore volume is 0.806 cm 3 g −1 for low O 2 -ow samples, which is an increase of 90% compared to 0.424 cm 3    The BET area was determined at 1094 m 2 g −1 for high O 2 -ow and at 2029 m 2 g −1 for low O 2 -ow, which are typical BET areas of KOH-activated carbons. 9,44,54Especially the direct comparison to the BET area of the pristine bres (13.4 m 2 g −1 ) clearly proves a successful KOH activation of the electrospun PAN-based carbon nanobres.Additionally, the high impact of the applied post-treatment conditions on the surface morphology and porosity of the carbon nanobres is obvious.
For more detailed micropore characterization CO 2 adsorption measurements at 273 K were performed.Fig. 8a shows the CO 2 adsorption isotherms for the pristine, 30 high O 2 -ow and the low O 2 -ow samples.The pristine bres isotherm exhibits a high uptake at low relative pressures and turns more into a linear shape at higher relative pressures a maximum CO 2 uptake of 2.7 mmol g −1 at 1 bar.In comparison, the adsorption isotherm of the high O 2 -ow shows a lower CO 2 uptake at pressures below 200 mbar.
At pressures above 200 mbar the adsorption isotherm of CO 2 surpasses the adsorption isotherm of the pristine bres and rises to 4.7 mmol g −1 at 1 bar (Fig. 8).Contrary, the low O 2 -ow samples exhibit a similar uptake as the pristine bres at low relative pressures and already surpasses the adsorption capacity of the pristine bres at pressures of 75 mbar.At 1 bar it reaches an CO 2 adsorption capacity of 6.5 mmol g −1 , which is 38% increase compared to the high O 2 -ow samples and even 140% increase compared to the pristine bres.
In comparison to literature data the obtained CO 2 adsorption capacities are among the highest for KOH-activated electrospun carbon nanobres.Wang et al. reported CO 2 adsorption capacities of 2.9 mmol g −1 at 1 bar and 273 K, which is signicantly lower than the reported values in the present work. 47Comparable CO 2 adsorption capacities were reported by Chiang et al. and Zainab et al. who obtained 3.5 mmol g −1 at 298 K, which is close to the obtained 4.2 mmol g −1 at 293 K on the high O 2 -ow sample in this study (S6).† 55,56 The pore size distribution was determined using Monte-Carlo calculations and the cumulative pore volume is shown in Fig. 8b.As described previously for DFT calculations based on Ar adsorption, the pore volume increases for low O 2 -ow samples over the full range of pore sizes compared to the high O 2 -ow samples.The total pore volume is 0.61 cm 3 g −1 for low O 2 -ow and 0.45 cm 3 g −1 for high O 2 -ow samples, resulting in an increase in pore volume of 36% for the low O 2 -ow samples.Regarding the ultramicropore volume (<0.7 nm), high O 2 -ow samples exhibit a value of 0.15 cm 3 g −1 , which is enhanced to 0.21 cm 3 g −1 on low O 2 -ow samples.Overall, the obtained pore volumes from CO 2 adsorption isotherms are comparable to those obtained from Argon isotherms, except for a signicant difference of the obtained micropore volume below 1.5 nm for DFT (0.365 cm 3 g −1 ) and Monte-Carlo (0.446 cm 3 g −1 ) data for high O 2 -ow.This deviation could be caused by the different surface chemistry of samples prepared at high and low O 2 -ow.High O 2 -ow samples exhibit a higher oxygen content than low O 2 -ow samples, which could affect the interactions of adsorptive and adsorbent.Such effects are not accounted for in V DFT(tot) (cm 3 g −1 ) V DFT<2nm (cm 3 g −1 ) V DFT<1.5nm(cm 3 g −1 ) V MC(tot) (cm 3 g −1 ) V DFT<0.7nm(cm 3 g −1 ) V MC<0.7nm (cm 3  the standard calculation models used for the determination of the pore size distributions.Furthermore, the isosteric enthalpy of adsorption was calculated for high and low O 2 -ow based on CO 2 adsorption isotherms measured at 273 K, 283 K and 293 K (S7).† For low O 2 -ow a value of 28.9 kJ mol −1 was determined at a loading of 0.1 mmol g −1 which slightly decreases to 27.1 kJ mol −1 at a loading of 6.5 mmol g −1 (Fig. 9).For high O 2 -ow, the obtained enthalpy of adsorption is at 27.4 kJ mol −1 at a loading of 0.1 mmol g −1 and decreases to 23.0 kJ mol −1 at a loading of 5 mmol g −1 .Both isosteric enthalpies of adsorption are comparable to adsorption enthalpies of activated electrospun carbon nanobres in literature. 57The more distinctive decrease of the isosteric enthalpy of adsorption for high O 2 -ow can be linked to a higher degree of surface oxygen due to the partial carbon oxidation at high O 2 -ow (Table 1), which probably lowers the binding affinity towards CO 2 due to the increased number of acidic groups on the surface.
To get an insight into the adsorption kinetics, equilibration curves of CO 2 at 50 mbar and 298 K were measured (Fig. 9b).For high and low O 2 -ow the adsorption rate is fast, as the equilibrium loading is reached within the rst 100 s with the steepest increase in the rst 20 s.Comparing the two posttreatments, there are no signicant changes of the adsorption kinetics.Therefore, the choice of post-treatment does not notably affect the adsorption rate of the carbon nanobres.
Overall, the low O 2 -ow conditions result in improved gas adsorption properties, namely a higher adsorption capacity and a higher micropore volume.Furthermore, the isosteric enthalpy of adsorption is increased for low O 2 -ow, whereas the adsorption kinetics are very similar for both post-treatments.Based on the results, the importance the choice of posttreatment conditions aer KOH activation of electrospun carbon nanobres becomes evident.

Conclusion
The effect of two atmospheric post-treatments aer the KOH activation of electrospun PAN-based carbon nanobers was  detailed studied with a focus on the occurring pyrophoric effect, chemical reactions during the post-treatment and changes of the morphology and adsorption properties.
At high O 2 -ow conditions a signicant formation of heat was observed and related to the oxidation reactions of metallic potassium, which was formed during the activation process prior to the post-treatment step.The reaction heat of the potassium oxidation reactions acts as igniter for a subsequent carbon combustion, which signicantly changes the pore structure and surface chemistry of the material and destroys the original bre structure.The comparability of high O 2 -ow to a usually applied ambient air post-treatment was shown in an additional experiment in which a signicant pyrophoric effect was visible.
Control of the vigorous potassium oxidation reactions was enabled by the application of a low O 2 -ow as post-treatment.The low O 2 -ow limits the oxidation reactions, resulting in a reduced heat formation, which is insufficient to cause a carbon combustion.A signicant increase in adsorption capacities and accessible pore volume as well as higher enthalpies of adsorption of CO 2 were found for low O 2 -ow samples.
Summarising, this work clearly shows that vigorous potassium oxidation reactions can occur aer the KOH activation of electrospun carbon nanobres and alter the obtained material.By proper choice of the atmospheric post-treatment conditions this potassium oxidation reactions can be limited and a significant improvement of the obtained porosity and surface chemistry can be achieved.

Fig. 1
Fig. 1 Pyrophoric KOH-activated electrospun PAN-based carbon nanofibres during exposure to ambient air directly after the simultaneous carbonisation and KOH activation.A red glowing of the carbon nanofibres can be observed (a Video of the glowing is attached as ESI †).

Fig. 2
Fig. 2 Temperature profile of the applied simultaneous carbonisation and activation in Ar with subsequent post-treatment at 4 mL min −1 (low O 2 -flow) and 175 mL min −1 O 2 -flow (high O 2 -flow).The vertical dashed line indicates the switch from Ar to N 2 : O 2 atmosphere.

Fig. 3
Fig. 3 (a) Mass and temperature changes during post-treatment at high O 2 -flow conditions (175 mL min −1 O 2 ).Average value of 5 measurements.The shaded areas show the standard deviation for the mass signal and the dashed line indicates the switch from Ar to N 2 : O 2 atmosphere.(b) DSC-TGA data during high O 2 -flow post-treatment of KOH-activated carbon nanofibres (O 2 -flow 175 mL min −1 ), where the dashed line indicates the atmosphere switch.A release of −2717 J g −1 reaction heat accompanied by a mass loss 8.2 wt.% is observed.
For the H and N content no signicant changes were obtained between high O 2 -ow and low O 2 -ow.Regarding the O content, an increase of 7 wt.% for high O 2 -ow compared to low O 2 -ow is observed.This is explained by the reduced amount of C relative to the O content and a partial oxidation of the carbon surface.However, the oxidation of the carbon surface would counteract the observed weight loss during the carbon combustion.For further structural characterization and comparison, Raman spectroscopy was performed on low and high O 2 ow samples.The results are shown and discussed in the ESI (S5).† 3.3.2Electron microscopy.As the described glowing and combustion effect on high O 2 -ow might cause severe changes to the surface morphology of the carbon nanobres, high O 2 -ow and low O 2 -ow samples were investigated with SEM

Fig. 4
Fig. 4 (a) Mass and temperature changes after the switch to low O 2flow (4 mL min −1 ) obtained as mean value of 5 measurements.The dashed line indicates the switch from Ar to N 2 : O 2 atmosphere and the shaded areas show the standard deviation for the mass signal.(b) DSC-TGA signal for low O 2 -flow post-treatment of activated carbon nanofibres based on 3 measurements.The dashed line indicates the switch to N 2 : O 2 atmosphere and the shaded areas show the standard deviation.After the switch a mass gain of 1.5 wt.% is observed accompanied by an emitted reaction heat of −94 J g −1 .

Fig. 5
Fig. 5 Molar amounts of CO 2 for high O 2 -flow and low O 2 -flow samples during post-treatment.Data obtained from MS measurements using a CaC 2 O 4 × H 2 O calibration.A CO 2 amount of 1 mmol g −1 for high O 2 -flow compared to 0.6 mmol g −1 for low O 2 -flow was detected.
g −1 for the high O 2 -ow.Such values are comparable to values obtained by Im et al. and Chiang et al. for PAN-based activated carbon bres.45,53

Fig. 6
Fig. 6 SEM images: (a) pristine carbon nanofibres shown as reference.(b) Low O 2 -flow samples without visible destruction of the fibre morphology.(c) High O 2 -flow samples with an area displaying a smaller degree of destruction (d) high O 2 -flow samples with severe destruction of the fibre morphology.

Fig. 7
Fig. 7 (a) Ar adsorption isotherms obtained at 87 K for high O 2 -flow and low O 2 -flow.Average isotherm of 5 measurements.(b) Cumulative pore size distribution for high O 2 -flow and low O 2 -flow calculated with a DFT kernel.Shaded areas show the standard deviation.

Fig. 9
Fig. 9 (a) Isosteric enthalpy of adsorption for high and low O 2 -flow calculated for CO 2 isotherms measured at 273 K, 283 K and 293 K.(b) Normalized adsorption kinetics for high and low O 2 -flow determined at 50 mbar and 298 K.

Fig. 8
Fig. 8 (a) CO 2 adsorption isotherms at 273 K for high O 2 -flow, low O 2flow and the pristine fibres.(b) Determined pore size distribution via Monte-Carlo calculations for pores smaller 1.5 nm.

Table 1
30NO elemental composition for neutralized high O 2 -flow, low O 2 -flow and pristine carbon nanofibres.O content is calculated as difference for high O 2 -flow and low O 2 -flow.Elemental composition of the pristine carbon nanofibres taken from.30Thevalue is the average of 5 measurements, the standard deviation is given as error g −1 )